This study established an experimental platform for nanometer‑precision tracking of individual sarcomere lengths (SLs) in rat neonatal cardiomyocytes by expressing AcGFP‑tagged α‑actinin at the Z‑discs. The system achieved spatial resolutions of 3 nm at 50 fps (and 8 nm even when Fluo‑4 was co‑imaged), revealing the diversity of sarcomere behavior at single‑cell, single‑sarcomere resolution—well beyond the reach of previous population‑averaged analyses. Real‑time observation of 100 nm SL changes enabled direct assessment of their impact on contractile force, shedding light on the micro‑mechanisms underlying the Frank–Starling law.
Key findings
SL changes propagate sequentially across the cell rather than occurring synchronously; relying on mean SL markedly underestimates shortening/lengthening velocities.
Spontaneous sarcomere oscillations (cell‑SPOC) induced by ionomycin and SR inhibition displayed waveforms essentially identical to those during electrical stimulation, supporting the idea that sarcomeres can act as autonomous oscillators.
The myosin activator omecamtiv mecarbil (OM) lowered oscillation frequency while increasing Z‑disc displacement and prolonging shortening time—consistent with the group’s SPOC model predicting higher myosin attachment and greater molecular friction, demonstrating the method’s utility for drug evaluation.
This nanometry approach provides a powerful tool for integrative, nanoscale understanding of excitation–contraction coupling. It later served as the experimental foundation for concepts such as Contraction Rhythm Homeostasis (CRH), Sarcomere Chaos with Changes in Calcium (S4C), and Chaordic Homeodynamics.
Article information & citation
Seine A. Shintani, Kotaro Oyama, Fuyu Kobirumaki‑Shimozawa, Takashi Ohki, Shin’ichi Ishiwata, Norio Fukuda. Sarcomere length nanometry in rat neonatal cardiomyocytes expressed with α‑actinin–AcGFP in Z discs. Journal of General Physiology 143, 513–524 (2014).